The scaling and time factor is a very difficult part of doing modern warfare modelling if you want overarching strategy in the same game that gives you low level tactical control.
Singular battles with well defined boundaries and durations aren't really 'a thing' anymore in large scale modern warfare. Where is the 'edge' of the battle? The artillery pieces miles off offering support? The opposing battery firing on your support artillery from even farther away? An airfield launching support craft? What about the military base on the other side of the globe currently loading transport aircraft with special forces paratroopers that are inbound to help?
If you have tactical control over a specific battle along with strategic control over the entire war, when what happens when you send a column of troops from one strategic location to the battle you're currently in tactical control of, and they get ambushed? Do you take tactical control there as well? Do you give it to the computer?
In short, if you're pushing things beyond the era of soldiers lining up to do volley fire across fields, then you're getting into the point of warfare becoming far too complex and fluid to allow a single person to keep even the remotest of grasps over the entire thing. It becomes too difficult to section it into manageable pieces that allow control and decisions to be made at all levels in a reasonable manner.
I would strongly suggest that anyone looking at modern military actions for a game setting to pick a military rank, and base a game around roughly what they actually control in the real world, and don't let the game expand beyond that focus. Playing as the commander of a platoon? Move squads around and change their formations. Commander of a company? Move platoons around and either abstract away the lower level movements or design an AI good enough to simulate someone else controlling them.
The higher up the command line you go, the larger the map scope and the more vague the fine details. If you're overseeing an entire global war, then you're not really all that worried about a specific pillbox or something somewhere in the world. You're worried that the front in the region is still advancing, and whether or not you're going to spend resources to reinforce it or not.